1. Field of the Invention
The present invention relates to a Ni-based alloy improved in oxidation-resistance, high temperature strength and hot workability, which alloy is suitable for making parts and members used at high temperatures exposed to an oxidation atmosphere, including automobile parts such as ignition plug electrodes, power plant facility's parts such as gas turbine nozzles, inner parts of heat treatment furnaces, and fuel cell's parts.
2. Description of the Related Art
Conventionally, a Ni-18Cr-7Fe alloy (Alloy 600), which has a high oxidation-resistance, has been used for parts which are exposed in an oxidation atmosphere at high temperatures. Oxidation-resistance of a material is required to prevent volume loss or embrittlement of the material due to oxidation while being used at high temperature in air or gas atmosphere. The oxidation-resistance of Alloy 600 is maintained because a Cr2O3 layer is formed on its surface at a high temperature thus protecting the base metal.
Recently, there is a growing need for various parts having, oxidation-resistance at higher temperatures than required for conventional applications, and studies to improve Alloy 600 have been conducted. Japanese Patent Laid-Open Nos. 63-153236 and 2000-336446 have proposed an improvement on Alloy 600 in oxidation-resistance. Japanese Patent Laid-Open Nos. 7-268522 and 11-12670 have proposed an Alloy 600 based alloy having improved high temperature strength for making members required to have high temperature strength.
According to the above described Japanese Patent Laid-Open No. 63-153236, Alloy 600 is added with Y, Ce, Zr, Sc and/or La to improve its oxidation-resistance. However, this alloy had a problem in hot workability and cracking occurred during hot working. Accordingly, Japanese Patent Laid-Open No. 2000-336446 proposed by some of the present inventors discloses that in order to achieve a good oxidation-resistant alloy, rare-earth elements, Y, Hf and/or Zr are added to a base alloy based on Alloy 600, the base alloy being made by adding Mg for improving hot workability and contains no Ti thereby having improved oxidation resistance. These alloys disclosed in Japanese Patent Laid-Open Nos. 63-153236 and 2000-336446 basically exhibited high oxidation-resistance at high temperatures. However, these alloys did not have sufficient high temperature strength.
The above described Japanese Patent Laid-Open No. 7-268522 has proposed an alloy of which temperature strength is improved by adding more than a predetermined amount of W and Mo. However, this alloy had a problem in hot workability and cracking occurred during hot working. According to Japanese Patent Laid-Open No. 11-12670, although high temperature strength was improved by adding a small amount of Nb, Mo and W, this alloy also had a problem in hot workability resulting in cracking during hot processing.
These problems concerning high temperature strength and hot workability of the alloy present a big challenge for providing parts and members, which are to be practically used in an oxidation atmosphere at high temperatures, including automobile parts such as ignition plug electrodes, power plant facility's parts such as gas turbine nozzles, inner parts of heat treatment furnaces, and fuel cell's parts.